We propose to characterize feline leukemia cell surface proteins associated with feline leukemia virus. The nature of cell surface molecules reactive with antisera to individual viral structural proteins will be determined. The reactivity of feline sera with such cell surface proteins will also be analyzed. Sera from healthy cats exposed to the virus in the natural environment, from exposed cats which develop lymphosarcoma, and from unexposed cats will be surveyed. Of particular interest will be characterization, on naturally occurring lymphosarcomas, of the feline oncornavirus-associated cell membrane antigen (FOCMA), which appears to be a tumor-specific antigen. Purification of FOCMA-carrying molecules will be attempted, as a step toward development of a vaccine against feline leukemia. In parallel studies we will compare the nature of FOCMA with that of human acute lymphocyctic leukemia cell antigens reactive with autologous antibodies from patients' sera.